Each summer as 120-degree heat shimmers silver waves across the Mojave Desert, residents of this inferno crank the thermostat down. Air-conditioner compressors and fan motors often run 24-hours a day and for weeks on end.

The problem is that many air conditioners in this torrid zone are huge power hogs. Which puts Steve Bouman in an unusual position. As head of energy efficiency programs for Mohave Electric, a small power cooperative based in Bullhead City, Ariz., he's got a front-row seat to one of the biggest campaigns for energy efficiency the nation has undertaken in years: high-efficiency air conditioning.

With the new plan, the United States should be able to keep cool while keeping energy consumption under control. Without it, the nation by 2020 would have to build more than 80 new power plants, creating about as much carbon annually as 3 million cars, according to one estimate. And it comes from a Republican administration, following a court battle in which it sought weaker standards.

So, without much fanfare, the US Department of Energy in April announced that all central home units (which use the lion's share of power, compared to just 12 percent for window units) sold in the US must achieve a "seasonal energy efficiency ratio" of 13 starting in January 2006. That's 30 percent more efficient than today's minimum standard of 10 SEER. (The SEER rating is calculated by dividing the total amount of heat removed from the air by the total energy required by the unit.)

Though the standard hasn't kicked in yet, manufacturers are racing to revamp production lines now and consumers are beginning to make the switch, starting with hot places like Bullhead City.

"We're not only in the desert, but the hottest part of the desert," says Mr. Bouman. "We're about 10 degrees hotter than Phoenix. Still, we have a lot of old-timers here who don't have AC. It's 110 to 115 degrees in their homes, but they're used to it."

Newcomers, on the other hand, aren't - and they're becoming more numerous. The region is in the midst of a building boom, fueled by Californians cashing in their pricey real estate to come to the high desert and build one, maybe even two, new homes - renting one out and living in the other. The boom puts Mohave Electric and its 34,000 customers in a quandary. More air conditioners mean more electricity sales but would require buying more costly "peaking power" or, worse, building an expensive new power plant.

That's why Bouman is on the road a lot these days, offering rebates to builders who will install high-efficiency air conditioning - and get others to retrofit homes with high-efficiency air conditioners or heat pumps. Still, energy efficiency can be a tough sell, since many homeowners flee to cooler climes in summer and so are unwilling to invest the hundreds extra to buy a super-efficient air conditioner.

Air conditioning uses only about 15.4 percent of all electricity delivered to homes annually, according to the Energy Information Administration, an arm of the DOE. But air conditioners use all of it in just a few weeks or months - and peak power loads can overstretch generating capacity and overheated power lines, causing blackouts like the one that knocked out power to millions across the eastern United States and Canada last August.

Indeed, air conditioning has a critical impact on peak power demand nationwide - as demonstrated in California, where residential and commercial air conditioning can account for up to 30 percent of the total power load during peaks, according to a Lawrence Berkeley National Laboratory study last year.

That's why energy forecasters keep a wary eye on air-conditioning growth. More than 60 million US homes - about 56 percent of the total - had central air conditioning in 2001, according to an analysis of most recent census data by the Air Conditioning and Refrigeration Institute, representing air-conditioning manufacturers. And that total has grown by some 5 million homes a year over the past decade - up from 39.3 million homes and a 42.2 percent share of all homes in 1991. Nearly 90 percent of new homes built in 2001 had central air.

Air-conditioning efficiency has been improving steadily since the 1970s energy crisis focused attention on power consumption. Homes are far more efficient and better insulated - and heating and cooling machinery is far more efficient. But more homes have air conditioning, and new homes are bigger - negating the gains made by more efficient equipment.

"We've had this situation where air- conditioning equipment has been getting more efficient, yet new homes are getting bigger - everything is supersized - so the result is you have more area to cool," says Andrew deLaski, executive director of the Appliance Standards Awareness Project, a Boston-based organization supported by a coalition of environmental and consumer groups, states, and utilities.

Power use for home air conditioning had been projected to rise 13 percent between 2002 and 2025 - without the new standards. That's modest compared with faster growing residential uses like personal computers, cell phone chargers, and other standby power energy "vampires," as President Bush has called them.

Yet because of its impact on peak power, growing demand for air conditioning is scarier than energy vampires and so must be kept under control, says Jonathan Koomey, a Lawrence Berkeley scientist who analyzes residential power use.

"It's true that power demand for residential cooling is growing at a moderate pace," he says. "But it's also true that air conditioning will be proportionally more important than other end uses because of the sort of impact it has on peak power demand."

By implementing the new standards, the nation would cut peak electric demand by 41,500 megawatts by 2020 and carbon emissions by some 7 million metric tons per year, according to an analysis by the American Council for an Energy-Efficient Economy (ACEEE) in Washington, an energy watchdog. That peak demand equals 83 500-megawatt power plants.

The standards are also projected to save consumers $3.4 billion on their electricity bills through 2020, the group says.

"These new standards for air conditioners represent a huge improvement in efficiency, with a lot of savings for consumers and less air pollution," says Steve Nadel, ACEEE executive director. "We're talking a big improvement in efficiency - and that's going to have a major impact."

Some manufacturers have been steadily pushing the technological envelope on efficient air conditioners for some time.

Sitting in his perfectly chilled office in steamy Tyler, Texas, Greg Walters spends a lot of time pondering cool ideas like: What's the most energy-efficient air conditioner my company can build?

Over a recent two-year period, Mr. Walters led an elite product-development team at Trane, a major air-conditioner manufacturer, in hot pursuit of the holy grail of air conditioners - a central-air unit for homes that would be 100 percent more energy efficient than anything the DOE currently requires.

Not surprisingly, the resulting Trane XL19i is pricey - a high-tech rocket ship among residential air conditioners that can cost up to 2.5 times a similar conventional unit. Despite its cost, its efficiency makes it popular even in cooler regions like New England, company officials say.

Yet for the nation's Sun Belt and its hottest regions like the Mojave, the XL19i is rated to use up to 95 percent less power than the government's air-conditioning energy standard. It may not be quite like putting a man on the moon, but the marriage of next-generation compressors, heat exchangers, coolant, and motor was plenty tough. "We had to use every resource we had to do it, absolutely," Walters recalls.

So far, the XL19i is selling well, he says. Sales of other manufacturers' super-high-efficiency models are also reportedly strong in the hottest regions of the country - Florida, Georgia, Texas, California - where air conditioners run around the clock. Rival Lennox Industries sells a 19.2 SEER unit. Tim Thorson, the senior product manager for cooling at Lennox, says its new model is selling well - even in Alaska.

"Certain people want not only the most efficient machine, but the one that's best at dealing with humidity and air quality," Mr. Thorson says.

Even in Oconomowoc, Wis., for example, about a third of replacement units are models in the 18 SEER range, says Jason Punzel, service and sales manager at Pat's Heating and Air Conditioning. "It does get pretty warm up here for a couple of months."